As an active hood, a hood is to be understood which, at an accident, is lifted automatically by means of one or more actuators to increase the distance between the hood and the parts of the vehicle which are difficult to deform so as to potentially provide an extended braking distance for a pedestrian impinging on the hood. Thus, it is intended to prevent that the pedestrian hits the non-deformable bodywork parts with a speed that can result in vital injuries.
Different pneumatic, in particular, pyrotechnic actuators have already been proposed to be used for this purpose such as the gas inflators, as they are known from air bags, to actuate the hood movement. These actuators have an advantageously short reaction time since they are generally electrically ignited and hence do not have to accelerate massive parts in a time-consuming manner for their activation, and they achieve a strong acceleration of the hood so that the latter reaches its lifted position in the shortest possible time. However, it is a disadvantage that the conventional actuators of this type are not reversible. This means, they are good for a single usage only and have to be replaced subsequently. A faulty activation of the actuators thus requires each time an expensive visit to the repair shop. Since it is hardly possible to avoid an activation of the actuators at an accident in case the hood is not affected by the accident, the repair costs after an accident for a vehicle equipped with pyrotechnic actuators are normally higher than the ones for a vehicle that does not have these actuators.
A further problem of the conventional pneumatic actuators is that, caused by their actuation, the stable support of the hood on the bodywork in its normal position is lost. After the actuation of the actuators, the hood is either loosely movable with respect to the bodywork and within the cylinder stroke of the actuators, or it is trapped in the lifted position. In both cases, to proceed driving after actuation of the actuators is afflicted with considerable safety risks, even if the vehicle, apart from that, is not substantially damaged, so that it is normally required to call qualified personnel to restart the vehicle.
Hence, there is a demand for a pneumatic actuator for an active hood which is capable, after an actuation, to move the hood automatically back into the normal position so that the roadworthiness of the vehicle can not be affected by the hood actuation alone.